1. Field of the Invention
This invention relates to a fluid pressure to electrical signal transmitter, and more particularly to an improved welded fluid tight seal between the diaphragm backup plate and fill fluid chamber.
2. Description of the Prior Art
Conventional prior art, pressure to current transmitters have four parts: namely, a process head arranged to receive into an internal cavity, a process fluid pressure under measurement, a flexible diaphragm and diaphragm backup plate used to form a flexible, fluid-tight side of the cavity within the process head, a center body having a fluid chamber located adjacent to the diaphragm and backup plate and containing a captive fluid through which pressure changes in the process fluid under measurement are transmitted from the diaphragm to a strain gauge in the fluid chamber, and a base portion arranged to protect electrical connections to the strain gauge for carrying a transmitter output signal to a remote measuring or controlling device. The four pieces of the process head are held in a fluid-tight stack by conventional nut and bolt connectors.
A resilient O-ring seal is employed in the aforementioned prior art devices between an outer face of the diaphragm backup plate and an outer mating face of the fluid chamber to prevent leakage of the captive fluid from the fluid chamber while allowing a pressure transmitting motion of the diaphragm.
Such an O-ring seal often suffers from several inherent problems, including non-uniform thickness defects, improper assembly producing defective seating and corrosion due to the chemical action which takes place between the captive fluid and the O-ring. These problems, either singly or in combination, can cause the O-ring to loose its proper sealing characteristic resulting in an undesired loss of fluid from the fluid chamber. This leakage, in turn, reduces the pressure that is applied by the captive liquid against its side of the diaphragm whereby the diaphragm will then have a tendency to be moved by the pressure of the process fluid under measurement into solid engagement with its backup plate. Since the diaphragm is then no longer in a flexible state it is not capable of accurately transmitting changes in pressure of the process fluid under measurement to the strain gauge.
Additionally, in prior art transmitters the diaphragm and diaphragm backup plate are often exposed to the process fluid which necessitates making them from a non-corrosive and expensive metal rather than made out of a mild steel since some process fluid under measurement can corrode mild steel. The use of such corrosion-resistant materials in combination with a mild-steel body for the transmitter is a source of further leakage at the interface using conventional seals due to the differential thermal expansion of such dissimilar materials.